In an injection molding machine, mold clamping force is usually exerted between a stationary platen and a movable platen during the clamping operation of the stationary mold and the movable mold, and a mold clamping frame receives reactive force of this mold clamping force. A known mold clamping frame (a so-called C-type frame) comprises a first member, a second member facing the first member, and a connecting member that links a lower end of the first member with a lower end of the second member. The first member is connected to a stationary platen, and the second member is connected to a movable platen. In this type of mold clamping frame, the reactive force of the mold clamping force is exerted on the first member and the second member, and bending moment is exerted on the connecting member. The connecting member bends due to the bending moment, and the first member and the second member are pushed open. It is consequently not possible to maintain the parallelism between the stationary platen and the movable platen. As a result, burrs are formed in the molded part.
Conventionally, the method of solving this problem has been to increase the rigidity of the connecting member and to suppress its bending deformation to a level that is not problematic. However, the problem with this method is that the mold clamping frame becomes larger and heavier. To deal with this, an injection molding machine has been developed to prevent the connecting member from bending without increasing the rigidity of the connecting member (e.g., Japanese Laid-open Patent Publication No. 8-294949).
A mold clamping frame of this injection molding machine has a first member, a second member facing the first member, and a connecting member connecting the center of the first member to the center of the second member (i.e. the mold locking frame is a so-called H-type frame). A stationary platen is fixed to an upper end of the first member, and a first cylinder is fixed to an upper end of the second member. A movable platen is fixed to a piston rod of the first cylinder. The movable platen is mounted such that it can slide on top of the connecting member. The first cylinder can cause the movable platen to advance or retreat with respect to the stationary platen. By contrast, a second cylinder is fixed to a lower end of the first member. A lower end of the second member is fixed to a piston rod of the second cylinder.
In this injection molding machine, the first cylinder exerts a mold clamping force between a movable mold and a stationary mold, and the second cylinder simultaneously exerts a balancing force on the first member and the second member. As a result, a reactive force to the mold clamping force is exerted on the upper ends of the first and second members, and the balancing force is exerted on the lower ends of the first and second members. Consequently, the bending moment that occurs in the connecting member due to the reactive force to the mold clamping force is cancelled out by the bending moment that occurs in the connecting member due to the balancing force. It is thus possible to prevent bending of the connecting member during the clamping operation of the stationary mold and the movable mold.
Although it is possible to prevent bending moment from being exerted on the connecting member in this conventional injection molding machine, bending moment is exerted on the first and second members, which causes bending deformation thereof. Compared to the bending deformation of the connecting member, the bending deformation of the first and second members has a smaller effect on the parallelism between the stationary platen and the movable platen, but cannot be ignored if the mold clamping force is greater. Further, there has been a demand in recent years for a smaller mold clamping frame and increased lightness in injection molding machines. When a mold clamping frame is made smaller and lighter, the bending rigidity of the first and second members consequently decreases, causing the bending deformation of the first and second members to become more severe. In the conventional injection molding machine, it becomes impossible to maintain the parallelism between the stationary platen and the movable platen when the bending deformation of the first and second members becomes more severe. Consequently, burrs are formed on the molded part.
Accordingly, one object of the present invention is to provide an injection molding machine capable of maintaining parallelism between two mold platens even if there is bending deformation of first and second members on which a reactive force to a mold clamping force is exerted.